1. Field of the Invention
The present invention relates to a system for controlling fuel vapor produced from the fuel tank, more particularly to a system for preventing the fuel vapor from being emitted to the atmosphere when the vehicle is refueled.
2. Description of the Prior Art
Automobiles emit various toxic matters such as carbonmonoxide (CO), nitrogenoxides (NOx), hydrocarbons (HC) and the like. Among those toxic matters, in recent years hydrocarbons, i.e., fuel vapor emitted from the fuel tank has come to public notice. It is said that the fuel vapor shares 25% of total hydrocarbons emitted from automobiles. The portion of fuel vapor generated when the vehicle is moving or stationary is called an evaporative emission. The control of the evaporative emission is a known technique which has been widely used in conventional vehicles.
On the other hand, the portion of fuel vapor which is generated when the vehicle is being refueled at a gas station is called a refueling vapor and it is emitted to the atmosphere during refueling through the fuel filler pipe unless controlled. The amount of the refueling vapor is almost equal to that of the fuel refueled. The inventor of the present invention proposes an apparatus for preventing the refueling vapor during refueling from being emitted outside the vehicle in Japanese Patent Application Laid-open No. Toku-Kai-Hei 8-258577.
FIG. 11 is a schematic diagram showing the construction of a fuel vapor emission preventing apparatus according to Toku-Kai-Hei 8-258577. The apparatus is constructed such that when a filler cap 21 is uncovered and then a fuel dispensing nozzle is placed in a filler pipe 20 to supply fuel to a fuel tank 10, the fuel dispensing nozzle guided by a restrictor 22 turns a shutter 23 to operate an air breather circuit changeover valve 63. This operation causes a shut-down of an air breather circuit 60 extending from the fuel tank 10 to a canister 30 through a fuel cut-off valve 61, a pipe 62, the air breather circuit changeover valve 63, a pressure control valve 64 and a roll-over valve 65 so as to prevent an over-filling.
The fuel vapor produced in the fuel tank 10 during refueling is guided through a pipe 74 to an air vent circuit changeover valve 72. Then, if the pressure in the fuel tank 10 is larger than the atmospheric pressure which is supplied through a pipe 73, the air vent circuit changeover valve 72 opens to guide the fuel vapor to the canister 30 wherein the fuel vapor is adsorbed.
However, the fuel vapor emission preventing apparatus of the prior art has such a disadvantage that if the pipe 73 is clogged by liquefied fuel vapor, the air vent circuit changeover valve 72 may not open successfully and this results in hindering refueling the fuel tank 10 due to an increased resistance of the air vent circuit changeover valve 72.